Environmental Engineering Reference
In-Depth Information
face water into the surface layer is provided by monsoon forcing (e.g. through
upwelling during the southwest monsoon and convective overturning during the
northeast monsoon). However, as a consequence of the non-uniformity of mon-
soon forcing in space and time, N
2
O distributions in the Arabian Sea surface
waters shows considerable seasonal and spatial variations. The highest surface
saturations have been found to occur during the southwest monsoon within
zones of coastal upwelling off Somalia (up to 330%; [30]), Oman (up to 230%,
Fig 7; [4]) and India (8250%; Naqvi et al., this volume). Widespread winter
cooling in the open northern and central Arabian Sea occasionally raises the sat-
uration level above 400% [47]. The lowest surface saturations (99-105%) have
been reported from the central Arabian Sea during the spring intermonsoon, a
period of strong vertical stability [4, 55].
Recently, Bange et al. [5] utilized over 2400 surface N
2
O measurements
made between 1977 and 1997 to compute mean seasonal and annual N
2
O
concentrations within 1
o
x1
o
grids. The annual emission rate calculated from
these data is 0.33-0.70 Tg N
2
O. As expected, coastal regions during the SW
monsoon appear to account for the bulk of the fluxes. However, this data set
did not include the very high N
2
O concentrations off the Indian west coast,
which suggest a seasonal (May-October) flux of N
2
O from the shelf ranging
from 0.06 to 0.39 Tg N
2
O [74]. Inclusion of these data would push up Bange et
al.'s [4] estimate to
0.4-1 Tg N
2
Oyr
−
1
, amounting to up to one-fifth of total
oceanic emissions of N
2
O (4.7 - 6.3 Tg N
2
Oyr
−
1
; [76]).
∼
5. ISOTOPIC CONSEQUENCES OF NITROGEN
CYCLING IN THE ODZ
There are two naturally occurring stable isotopes of nitrogen,
14
N and
15
N,
with
14
N far more abundant, 99.63%. Because of this mass difference, the
activation energy for biological reactions involving the heavier isotope,
15
N, is
slightly greater than that for the lighter isotope, resulting in a slower reaction.
Consequently, there occurs a kinetic discrimination against the heavier isotope
leading to an increase in the
15
N/
14
N ratio in the reactants and a decrease in
the ratio in the products. The isotope composition is typically expressed as the
part per thousand change in the isotope ratio, δ = ‰, relative to a standard; in
the case of nitrogen the standard is air. These isotopic fractionations are very
useful in the study of nitrogen cycling in the oceans.
Isotope measurements have been made for three of the major nitrogen species
involved in nitrogen cycling in the Arabian Sea ODZ [8, 72, 73, 98]. At a station
in the heart of the ODZ with a well developed SNM, the isotopic composition
of NO
3
−
shows a dramatic increase in δ
15
N (Fig. 8). Outside the ODZ's the
isotopic composition of NO
3
−
is relatively constant (4-6‰; [86, 92]). Deep-
water δ
15
N
NO
3
values in the Arabian Sea are within this range. However, as
one moves up in the water column the δ
−
15
NofNO
3
−
increases dramatically to
